1. Technical Field
The present invention relates to a method and structure for coupling a heat sink or heat spreader to a semiconductor chip.
2. Related Art
FIG. 1 illustrates an electronic package 10 with a semiconductor chip 14 coupled to a chip carrier 12. A heat spreader 16 is coupled to the chip 14 with an interfacing encapsulant 20 between the heat spreader 16 and the chip 14. If conductive fins 22 are present, then a heat sink comprises a composite of the heat spreader 16 and the conductive fins 22. Although the discussion infra in this Related Art section discusses only the heat spreader 16, it should be understood that the heat sink may be present and is analogous to the heat spreader 16. The encapsulant 20 serves to couple the heat spreader 16 to both the chip 14 and the chip carrier 12. A layer 21 of the encapsulant 20 between the chip 14 and the heat spreader 16 has a constant thickness (t), wherein t is in a direction that is normal to a surface 15 of the chip 14. An underfill 18 relieves thermally induced stresses, as well as consequent premature fatigue failure, that might otherwise be imposed on solder connections between the chip 14 and the chip carrier 12 due to differential thermal expansion between the chip 14 and the chip carrier 12. The differential thermal expansion is a consequence of a coefficient of thermal expansion (CTE) differences between the chip 14 (e.g., 3 to 6 ppm/.degree. C.) and the chip carrier 12 (e.g., 10 to 24 ppm/.degree. C. for an organic chip carrier; 6 to 10 ppm/.degree. C. for a ceramic chip carrier).
The encapsulant 20 typically has a much higher CTE (e.g., 17 to 70 ppm/.degree. C.) than a CTE of the chip 14 (e.g., 3 to 6 ppm/.degree. C.) which causes the encapsulant 20 to swell or contract more 5 than the chip 14 when the electronic structure 10 is heated or cooled, respectively. As the encapsulant 20 swells, the encapsulant 20 expands away from the chip 14 and lifts the heat spreader 16 away from the chip 14, which causes high thermally induced stresses at the interfaces with both the chip 14 and the heat spreader 16. Because of the aforementioned thermally induced stresses, the chip 14 or the heat spreader 16 may delaminate from the encapsulant 20, or a crack in the encapsulant 20 may form and propagate, with an accompanying loss of structural integrity and/or degradation of heat transfer capability. The thermally induced stresses at the interfaces of the chip 14 and the heat spreader 16 to the encapsulant 20 are highest near peripheral edges 24 of the chip 14.
A method that reduces interfacial thermally induced stresses and does not materially increase thermal resistance is needed for coupling a heat spreader or a heat sink to a chip.